Pelagic trawling is the use of a cone-shaped net behind a boat to catch large schools of fish. It differs from benthic trawling—sometimes referred to as bottom trawling—in that as opposed to towing trawl nets along the sea floor, pelagic trawling provides for a towing higher up in the water column. In both trawling techniques, a flow of water passes through the net as the net is towed behind the fishing vessel.
A fishing trawl may be considered as having two main portions: the trawl-net portion which is the extended area of netting that serves to capture a large volume and which tapers inwardly towards a collecting bag or cod-end, where the captured fish are retained until the trawl is recovered on board the fishing vessel where they are then released and processed. The body of the trawl-net is funnel-like, wide at its mouth and narrowing towards the cod-end. It is long enough to assure adequate flow of water and prevent fish from escaping the net after subsequent capture. It is made of different grades of netting, the size of the mesh of the netting decreasing from the front of the net towards the cod-end. The cod end is where fish are finally caught. The size of mesh in the cod end is a determinant of the size of fish which the net catches.
Within the context of large scale trawling arrangement it is known to provide selective species discrimination to ensure for example that a fish capture is predominately the desired species. This is typically achieved by providing escape arrangements in the body of the trawl-net and is known for example in the context of tuna fishing to ensure that large populations of dolphin are not captured when the intended capture is tuna. While it is known to species select it is also known to grade the species being caught as well. Such fish grading is provided to attempt to minimise the numbers of juvenile fish that are caught within the trawl. The fish grading is achieved by controlling the sizes of mesh in the cod-end.
Such grading of species is important in modern fish management where there are strict controls on the volumes of fish caught and also on the minimum size of the catch. The use of larger mesh sizes in the collecting bag (cod end) was among the first technical measures imposed by fisheries managers to prevent the capture of juveniles. Such arrangements require the fish to reach the side netting to achieve their escape.
Other arrangements provide for selective dimensioning of portions of the collecting bag to attempt to create areas within the cod end where such juvenile species may escape. Such arrangements typically are arranged for small volumes of fish and are generally rigid in construction. Examples of such rigid constructions would include WO9217999 and U.S. Pat. No. 6,223,462 which both employ rigid grids. The rigidity of such structures is problematic both in scaling the size of the net arrangement and also when casting and retrieving the nets back on board the fishing vessel as it is important to ensure that the retrieved netting can be processed through the fishing gear and then stowed safely in a relatively small volume. The rigid structure of such known arrangements hinders this processing. There is therefore a need to provide a trawl netting arrangement that allows for selective discrimination in favour of a release of juvenile species, is flexible enough to be wound on to a net drum and also robust enough to withstand winding on to a net drum. GB2353930 describes an arrangement in which a grid array is provided from a plurality of loops. Each loop in the grid array includes tubes threaded on a netting line such that while the minimum dimensions of each of the loops are fixed, the minimum separation being defined by the length of each of the tubes, each of the tubes are pivotable on their netting line with respect to adjoining tubes, which is described as providing an improvement in handling compared to, for example, the previously described rigid steel grids. WO98/36638 describes a size-sorting trawl including a grille which provides for a selective preferential capture of smaller fishes whereas larger fishes are discriminated against. Such an arrangement is disadvantageous in pelagic fishing such as for herring or mackerel where volumes of fish of about 400 tonnes passing through the net in about 4-5 minutes is not unusual. If an arrangement such as that described in WO98/36638 was provided, the grille would provide an obstruction within the net with potential catastrophic results.
While the above described arrangements provide for a certain degree in selection in the size of the captured fish, there is a further need to provide a netting arrangement that provides for discriminative capture of the fish size. It is believed that whilst all of the above described arrangements improve the prospect of escape by juvenile species, there is still a need for an improved netting arrangement.